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United States Patent |
5,610,694
|
Lior
,   et al.
|
March 11, 1997
|
Latent development apparatus for use in electrophotographic imaging
system
Abstract
Imaging apparatus including:
an image-forming surface having formed thereon a latent electrostatic
image, the latent electrostatic image including image regions at a first
voltage and background regions at a second voltage;
a developer surface charged to a third voltage intermediate the first and
second voltages and having a second surface urged against and adapted for
operative engagement with the image-forming surface at a development
region;
an applicator assembly operative to apply a layer of concentrated liquid
toner comprising charged toner particles and carrier liquid onto the
developer surface;
a developer operative to develop the latent image by transferring less than
the total thickness of the layer of concentrated liquid toner from the
developer surface to the image regions of the image-forming surface
thereby to form a developed image on the image-forming surface; and
apparatus for transferring the developed image from the image-forming
surface to a final substrate.
Inventors:
|
Lior; Ishaiau (Ness Zlona, IL);
Lavon; Amiran (Bat Yam, IL);
Chatow; Ehud (Petach Tikva, IL)
|
Assignee:
|
Indigo N.V. (SM Veldhoven, NL)
|
Appl. No.:
|
464851 |
Filed:
|
August 17, 1995 |
PCT Filed:
|
January 11, 1993
|
PCT NO:
|
PCT/NL93/00010
|
371 Date:
|
August 17, 1995
|
102(e) Date:
|
August 17, 1995
|
PCT PUB.NO.:
|
WO94/16364 |
PCT PUB. Date:
|
July 21, 1994 |
Current U.S. Class: |
399/240 |
Intern'l Class: |
G03G 015/10 |
Field of Search: |
355/256
188/661,652
|
References Cited
U.S. Patent Documents
3921579 | Nov., 1975 | Wright | 118/651.
|
4327664 | May., 1982 | Ohkawa et al. | 118/661.
|
4400079 | Aug., 1983 | Landa | 355/256.
|
4504138 | Mar., 1985 | Kuehnle et al. | 355/256.
|
4684238 | Aug., 1987 | Till et al. | 355/256.
|
4974027 | Nov., 1990 | Landa et al. | 355/256.
|
4984025 | Jan., 1991 | Landa et al. | 355/274.
|
4999677 | Mar., 1991 | Landa et al. | 355/273.
|
5028964 | Jul., 1991 | Landa et al. | 355/273.
|
5047808 | Sep., 1991 | Landa et al. | 355/277.
|
5089856 | Feb., 1992 | Landa et al. | 355/279.
|
5436706 | Jul., 1995 | Landa et al. | 355/256.
|
Foreign Patent Documents |
990589 | Jun., 1976 | CA.
| |
9004216 | Apr., 1990 | WO.
| |
Primary Examiner: Grimley; Arthur T.
Assistant Examiner: Grainger; Quana
Attorney, Agent or Firm: Greenblum & Bernstein P.L.C.
Claims
We claim:
1. Imaging apparatus comprising:
an image-forming surface having formed thereon a latent electrostatic
image, the latent electrostatic image including image regions at a first
voltage and background regions at a second voltage;
a developer surface charged to a third voltage intermediate the first and
second voltages and having a second surface urged against and adapted for
operative engagement with the image-forming surface at a first,
development, region, said developer surface capable of having formed
thereon a layer of concentrated liquid toner comprising charged toner
particles and carrier liquid;
a developer comprising the developer surface, which develops the latent
image, wherein the third voltage is such that less than the total
thickness of the layer of concentrated liquid toner is transferred from
the developer surface to the image regions of the image-forming surface to
form a developed image on the image-forming surface; and
means for transferring the developed image from the image-forming surface
to a final substrate.
2. Apparatus according to claim 1 wherein the developer comprises a
developer roller electrified to a given potential such that the electric
field which is produced by the developer roller voltage, the image area
voltage and the charge in the toner layer reverses direction within the
layer.
3. Apparatus according to claim 2 wherein the voltage difference between
the developer roller and the image region is less than 500 volts.
4. Apparatus according to claim 2 wherein the voltage difference between
the developer roller and the image region is more than 300 volts.
5. Apparatus according to claim 1 including an applicator assembly for
forming the layer of concentrated liquid toner onto the developer surface,
the applicator assembly comprising:
an applicator having a moving surface which is urged into resilient
engagement with the developer surface at a second, application, region;
a toner dispenser which supplies liquid toner comprising charged toner
particles and carrier liquid to the applicator;
a power supply which electrifies the applicator with a voltage such that
the electric field formed by the difference in voltage between the
developer surface and the applicator in the application region urges the
charged toner particles toward the developer surface, forming a layer of
charged liquid toner concentrate on the developer surface, wherein
the applicator and the developer surface have substantial relative movement
at the application region.
6. Imaging apparatus according to claim 5 wherein the concentration of
solids in the liquid toner supplied to the applicator assembly is less
than 10 percent and the concentration of solids in toned portions of the
developed image is at least 25 percent.
7. Imaging apparatus according to claim 5 and further comprising an
electrified squeegee roller, which engages the developer surface at a
squeegee region situated downstream of the application region and before
the development region and squeegees the layer of concentrated liquid
toner and increases the concentration of liquid toner therein.
8. Apparatus according to claim 5 and further comprising a cleaning station
which removes residual toner from the developer surface after development
of the latent image and before application of new toner by the applicator
assembly.
9. Apparatus according to claim 8 and wherein the cleaning station
comprises a sponge roller which engages with the developer surface at a
cleaning region downstream of the development region.
10. Apparatus according to claim 8 wherein the cleaning station comprises;
a squeegee roller urged against the developer surface and biased to a
voltage at which the particles are attracted to the squeegee roller; and
a sponge roller urged against the squeegee roller and moving at a
differential velocity with respect to the adjacent surface of the squeegee
roller.
11. Imaging apparatus according to claim 1 wherein the toner dispenser
comprises a perforated dispenser tube urged against the applicator at a
region remote from the application region.
12. Apparatus according to claim 1 including an applicator assembly which
forms the layer of concentrated liquid toner onto the developer surface,
the applicator assembly comprising a stationary plate-type developer
situated adjacent to the developer surface and includes means for
supplying liquid toner to an application region formed between the plate
and the developer surface.
13. Apparatus according to claim 1 wherein the concentration of solids in
the image portions of the developed image is at least 30 percent.
14. Apparatus according to claim 1 wherein the concentration of solids in
the image portions of the developed image is at least 40 percent.
15. Imaging apparatus comprising:
an image-forming surface having formed thereon a latent electrostatic
image, the latent electrostatic image including image regions at a first
voltage and background regions at a second voltage;
a developer surface charged to a third voltage intermediate the first and
second voltages and having a second surface urged against and adapted for
operative engagement with the image-forming surface at a first,
development, region;
an applicator assembly operative to supply a layer of concentrated liquid
toner comprising charged toner particles and carrier liquid onto the
developer surface, the applicator assembly comprising:
an applicator having a moving surface which is urged into resilient
engagement with the developer surface at an application, region;
a toner dispenser which supplies liquid toner comprising charged toner
particles and carrier liquid to the application region;
a power supply which electrifies the applicator with a voltage such that
the electric field formed by the difference in voltage between the
developer surface and the applicator at the application region urges the
charged toner particles toward the developer surface, forming a layer of
charged liquid toner concentrate on the developer surface, wherein
the applicator and the developer have substantial relative motion at the
application region; and
wherein the developer surface develops the latent image by transferring at
least a portion of the total thickness of the layer of concentrated liquid
toner from the developer surface to the image regions of the image-forming
surface to form a developed image on the image-forming surface; and
means for transferring the developed image from the image forming surface
to a final substrate.
16. Apparatus according to claim 15 wherein the applicator is a sponge
roller comprised of open cell pores.
17. Apparatus according to claim 15 wherein the applicator and the
developer surface move in opposite directions at the application region.
18. Apparatus according to claim 17 wherein the relative motion of the
applicator and developer surface is between three and four times the
absolute velocity of the developer surface.
19. Apparatus according to claim 15 wherein the relative velocity between
the developer surface and the applicator is greater than 150 mm/sec.
20. Cleaning apparatus for removing residual charged liquid toner particles
from a moving surface comprising:
a squeegee roller urged against the surface and biased to a voltage at
which the particles are attracted to the squeegee roller; and
a sponge roller urged against the squeegee roller and moving at a
differential velocity with respect to the adjacent surface of the squeegee
roller.
21. Apparatus according to claim 20 wherein the sponge roller is biased to
a voltage at which the toner particles are attracted thereto.
22. Apparatus according to claim 20 and also including means for supplying
a liquid to the interface between the squeegee roller and the surface to
be cleaned.
23. Apparatus according to claim 20 and also including means for supplying
a liquid to the interface between the squeegee roller and the sponge
roller.
Description
FIELD OF THE INVENTION
The present invention relates to development apparatus in general and, more
particularly, to latent image development apparatus in electrophotographic
imaging systems.
BACKGROUND OF THE INVENTION
The method of developing a latent image formed on a photoconductive surface
by means of electrophoretic transfer of liquid toner is well known in the
art. In this method, charged particles suspended in a non-polar insulating
carrier liquid migrate under the influence of an electrostatic field and
concentrate in an image forming configuration upon relatively charged or
discharged areas of a photoconductive surface. The developed image is then
transferred to a substrate, such as paper, either directly or by means of
one or more intermediate transfer members.
In U.S. Pat. No. 4,504,138 a different method for the developing of a
latent image is described. This patent describes applying a thin viscous
high density layer of toner particles on the circumferential surface of a
roller and bringing the layer so formed to the photoconductive surface.
Transfer of selected portions of the toner layer onto the photoconductive
surface then occurs due to the electric field induced by the latent image.
In Canadian Patent 990589, a method of developing electrostatic images is
described which involves producing a film of liquid toner on a first
applicator and bringing the applicator in contact with the final substrate
which carries a latent image, thereby to develop the image. A second
applicator bearing a layer of carrier liquid is then brought into contact
with the substrate to remove background deposits and to squeegee out
excess liquid. The film of liquid toner described in Canadian Patent
990589 has between 2-4 per cent of toner concentrate dispersed within the
carrier liquid.
A latent image development apparatus described in U.S. Pat. No. 4,327,664
includes a porous, resilient sponge, development roller which is
circumferentially surrounded by a net of fine mesh size. The developer
roller is urged against the latent image carrying surface of a drum and
liquid toner, which is carried in the roller is squeezed out of the
compressed sponge through the fine net. Toner particles which are
contained in the liquid toner are selectively deposited, by
electrophoresis, onto the surface of the drum to form an image.
U.S. Pat. No. 4,400,079 describes a liquid toner development system for
developing a latent image on a photoreceptor that uses a non-contacting
developer roller. The development roller surface may move in the same
direction as the photoreceptor surface or in the opposite direction.
Most of the above mentioned apparatus, as well as many other techniques
which are known in the art, are concerned with producing a layer of toner
on the surface of a developing roller and transferring the entire
thickness of the layer to image of a latent image on an image bearing
substrate. For such systems, the uniformity of the layer thickness is
important, since this thickness determines the density of the image.
Normally, it is very difficult to control the uniformity of the developing
layer even by expensive and complicated means, especially when the layer
thickness is generally a function of the previous imaging history of the
apparatus.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide simplified apparatus
for the development of latent images in electrophotographic imaging
systems by transfer of concentrated liquid toner, wherein the optical
density of toner in the toned regions of the final image is substantially
uniform.
In preferred embodiments of the present invention a developer roller,
preferably made of resilient material, is urged against an image forming
surface, such as a photoreceptor, that has an electrostatic latent image
comprising image areas at a first voltage and background areas at a second
voltage formed thereon. The developer roller is coated with a relatively
thin film of concentrated liquid toner material having a given layer
thickness. At least a portion of this layer thickness is selectively
transferred to the image-forming surface in accordance with the latent
image formed thereon.
According to one aspect of the invention, the developer is electrified to a
voltage which is intermediate the first and second voltages. In a
preferred embodiment of the invention, the developer voltage is selected
to cause only a portion of the layer thickness to transfer to the image
areas of the latent image. The present inventors have found that when the
developer voltage is properly chosen, the density of toner particles per
unit area (DMA) is less strongly dependent on the thickness of the toner
concentrate layer on the developer roller or on the layer's solids
concentration. Thus, even if the thickness of the layer on the developer
roller varies by an unacceptable amount, the non-uniformity of the layer
transferred to the image forming surface is improved at least by a factor
of two.
In a second aspect of the invention, the thickness uniformity of the toner
concentrate layer on the developer is improved by supplying the toner
concentrate preferably from a sponge roller which is urged against the
developer roller and which travels at a different speed from that of the
developer roller at the point of contact between the two rollers.
Increased uniformity results even if the toner concentrate remaining on
the developer roller is not removed from the roller before application of
a new layer of toner concentrate. When the applicator roller and developer
rollers move at the same speed at the point of contact, there is a
substantial "memory" of the prior image which non-uniformly effects the
developed toner particle mass per unit area (DMA) in the layer on the
developer roller.
In a preferred embodiment of the invention, the concentration of toner
particles on the layer is increased by application of an electrified
squeegee roller to the layer before the transfer to the imaging surface.
In a preferred embodiment of the invention, the applicator roller moves in
a direction opposite to that of the developer roller at the point of
contact between them.
In one preferred embodiment of the invention, the developer roller is
coated by developing thereon a layer of toner particles by electrophoresis
from liquid toner using a stationary plate type developer.
Preferably, the coating step is followed by squeegeeing the layer on the
developer roller with a squeegee roller at a high voltage and with high
pressure in order to remove a large portion of the liquid therein, before
transfer of all or a portion of the resultant thickness of the layer to
the image forming surface.
Further, material remaining on the developer roller after development of
the latent image is preferably removed in a cleaning step before the
recoating of the developer roller.
In a preferred embodiment of the invention, the developer roller may be
cleaned by a tandem roller system in which a first, biased squeegee,
roller removes the toner particles from the developer roller by
electrostatic transfer. A second, sponge cleaning, roller removes the
toner particles from the biased squeegee roller by abrasion, preferably
aided by an electrostatic field. The toner is removed from the sponge
roller by a blade which indents the surface of the sponge and scrapes the
material which is extruded away. Preferably, one or both of the
squeegee-developer or squeegee-sponge surfaces is wetted by a cleaning
liquid or dilute toner to aid in transfer and removal of the toner
particles. The toner so removed is preferably recycled and used for
recoating the developer roller.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood and appreciated more fully from
the following detailed description, taken in conjunction with the drawings
in which:
FIG. 1 is a schematic diagram of imaging apparatus constructed and operated
in accordance with a preferred embodiment of the present invention; and
FIG. 2 is a more detailed schematic diagram of a developer assembly
constructed and operated in accordance with a preferred embodiment of the
present invention.
FIG. 3 is a schematic diagram of a developer assembly illustrating an
alternative cleaning system in accordance with a preferred embodiment of
the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS.
Reference is made to FIGS. 1 and 2 which illustrate imaging apparatus
constructed and operative in accordance with a preferred embodiment of the
present invention.
The apparatus includes a drum 10 preferably having a cylindrical
photoreceptor surface 16 made of selenium, a selenium compound, an organic
photoconductor or any other suitable photoconductor known in the art.
During operation, drum 10 rotates in the direction indicated by arrow 14
and photoreceptor surface 16 is charged by a charger 18 to a generally
uniformly predetermined voltage, typically on the order of 1000 volts.
Charger 18 may be any type of charger known in the art, such as a
corotron, a scorotron or a roller.
Continued rotation of drum 10 brings charged photoreceptor surface 16 into
image receiving relationship with an exposure device such as a light
source 19, which may be a laser scanner (in the case of a printer) or the
projection of an original (in the case of a photocopier). Light source 19
forms a desired latent image on charged photoreceptor surface 16 by
selectively discharging a portion of the photoreceptor surface, the image
portions being at a first voltage and the background portions at a second
voltage. The discharged portions preferably have a voltage of less than
about 100 volts.
Continued rotation of drum 10 brings the selectively charged photoreceptor
surface 16 into operative contact engagement with a surface 21 of a
developer roller 22. Developer roller 22 preferably rotates in a sense
opposite that of drum 10, as shown by arrow 13, such that there is
substantially zero relative motion between their respective surfaces at
the point of contact. Developer roller 22 is most preferably urged against
drum 10.
In one embodiment of the invention, developer roller 22 is formed with a
metal core coated with, preferably, 1-2 mm of a soft elastomer material
having a Shore A hardness of preferably 20-40. In one embodiment of the
invention, this coating is made conductive, preferably, to a resistivity
between 10.sup.5 and 10.sup.7 ohm-cm. In this embodiment, the conductive
layer is either covered with a smooth elastomer layer or is formed by
casting or other process to have a smooth surface, preferably better than
N3.
In a second embodiment of the invention, the soft elastomer material, which
may be non-conductive, is coated with a very thin electroconductive layer,
such as for example a metal or conducting lacquer layer, which is
electrically attached to the metal core. This conductive layer is
preferably covered by a thin (preferably 15 to 60 micrometer) layer of
conducting polymer having a resistivity, preferably, between 10.sup.7 to
10.sup.9 ohm-cm.
Alternatively, drum 10 may be formed of a relatively resilient material,
and surface 21 may be composed of either a rigid or compliant material.
As described below, surface 21 is coated with a thin layer of liquid toner,
which is preferably very highly concentrated liquid toner. Developer
roller 22 itself is charged to a voltage which is intermediate the voltage
of the charged and discharged areas on photoreceptor surface 16.
When surface 21 bearing the layer of liquid toner concentrate is engaged
with photoreceptor surface 16 of drum 10, the difference in potential
between developer roller 22 and surface 16 causes selective transfer of
the layer of toner particles to surface 16, thereby developing the latent
image. Depending on the choice of toner charge polarity and the use of a
"write-white" or "write-black" system as known in the art, the layer of
toner particles will be selectively attracted to either the charged or
discharged areas of surface 16, and the remaining portions of the toner
layer will continue to adhere to surface 21.
When liquid toner having a very high concentration of solids at development
is used, as in the preferred embodiments of the present invention, there
is little if any electrophoresis and the entire thickness of the layer or
a controllable portion of the thickness is transferred to the image areas
of the latent image at substantially the same toner concentration as the
layer on the developer surface.
In an alternative, especially preferred, embodiment of the invention, the
voltage difference between the image portions of the latent image and the
developer roller is reduced to a value at which only part of the thickness
of the toner concentrate layer is transferred from the developer roller to
the image portions of the latent image.
The thickness of the layer that is transferred depends mainly on the charge
per unit volume in the layer and depends only slightly on the local
thickness or solids concentration of the layer. Thus, even if the layer on
the developer roller is not uniform, the layer transferred to the image
areas has a uniform DMA and thus a uniform optical density.
This phenomenon is probably due to the following effect: When a voltage is
applied across the toner concentrate layer between the development roller
and the image areas on the image forming surface, an electric field
between the two surfaces is generated. This field is strongest at the
image-forming surface and is reduced by the charge in the layer, at points
within the layer itself. When a high voltage is applied to the toner
concentrate layer, the electric field within the layer is unidirectional,
and acts to urge the entire toner layer toward the imaging surface at
image regions. When the impressed voltage is low, the electric field may
reverse at some point in the layer. For those portions of the layer
between this point and the developer roller the electric field actually
acts to force the toner particles toward the developer roller.
If the cohesiveness of the layer is not too high, the layer will split at
or near this point, with part of the toner layer being transferred to the
image-forming surface and part remaining on the developer roller. The
amount of solids which is transferred is dependent only on the charge per
unit mass in the layer and is not a strong function of the layer thickness
or the exact concentration of toner particles in the layer. Thus,
development system of the present invention is seen to be substantially
less sensitive to variations in application parameters which are difficult
to control.
Furthermore, since the DMA is basically directly proportional to the
voltage difference applied, the DMA can be easily controlled by changing
this voltage. In particular, in a preferred embodiment of the invention, a
sensor 100, as known in the art, is placed on the image forming surface
downstream of the development region to measure the optical density of the
image (and hence the DMA).
The measured value of optical density is then supplied to control
electronics 102 which is operative to control a power supply 104 which
supplies voltage to developer roller 22.
If the preferred liquid toner of the invention is utilized at a
concentration of 25-30% toner particles on the developer layer at a DMA of
0.2 mgm/cm.sup.2 (with variations of between about 0.17 and 0.25
mgm/cm.sup.2), the transferred layer will preferably be about 0.15
mgm/cm.sup.2 with worst case variations of less than .+-.10%.
In this situation a voltage difference between the image portion and the
developer roller of 500-600 volts will result in complete transfer of the
layer, while a voltage difference of between 300 and 500 volts will result
in the above-described more uniform partial transfer at a process speed of
50 mm/sec.
The use of such partial development dramatically reduces the effect of
possible non-uniformities in the toner layer on surface 21. Specifically,
the present inventors have found that a layer non-uniformity of
approximately .+-.20 (i.e. a difference of up to .+-.20% in toner layer
density between different areas) on developer surface 21 may be reduced,
by using partial development, to a non-uniformity of substantially less
than .+-.10% on the image portions of the latent image on photoreceptor
surface 16. For lower initial non-uniformities, for example .+-.5%, the
non-uniformities are reduced to the 1-2% range.
This process results in an image of more uniform density and is especially
useful for half-tone imaging in which the image density is to be
controlled by the percentage of area to be printed.
Other liquid toner concentrations, as high as 40-50%, with the preferred or
other toner materials, can also be used in the practice of the invention
for the layer on the developer. For these other toners, other
concentrations or for different process speeds, the exact required voltage
is determined by experimentation.
For multicolor systems, a plurality of developer rollers may be provided,
one for each color, spaced circumferentially around the photoreceptor
which are sequentially engaged with photoreceptor surface 16 to develop
sequentially-produced latent images.
In a preferred embodiment of the invention, roller 22 is coated by an
applicator assembly generally indicated by reference numeral 23.
Applicator may be an assembly as shown in more detail in FIGS. 2 and 3, or
may be a stationary plate type developer. The toner is coated onto the
developer by electrophoresis.
Applicator assembly 23 includes a toner dispenser 62 which can be
visualized as having the shape of a flute, i.e. a cylindrical tube having
a plurality of holes at different locations along its longitudinal
dimension, through which liquid toner is dispensed onto the surface 64 of
an applicator roller 65. Applicator 65 is preferably formed of a metal
core 66 covered with a relatively thick layer 68 of a resilient open-cell
foam (sponge), such as foamed polyurethane, which preferably contains
conductive additives. Preferably, the bulk resistivity of the polyurethane
(without the holes) is between 10.sup.7 and 10.sup.9 ohm-cm. In a
preferred embodiment, surface 64 of applicator roller 65 is resiliently
urged against surface 21 of developer 22, for example, by virtue of a
spring 72 which acts upon the ends of core 66.
As can be seen in FIG. 2, dispenser 62 is preferably forced into applicator
roller 65 such that a depression is formed in the outer portion of
applicator roller 65, thereby deforming surface 64. The continuous
deformation of surface 64 and compression of layer 68 is operative to open
cells of layer 68 which may occasionally clog during the operation of
developer assembly 23 and the successive release of the deformation is
operative to fill the cells. Most of the toner dispensed from dispenser 62
is rapidly absorbed by layer 68 of applicator roller 65, and is
homogeneously distributed within layer 68 due to the spongy open-cell
structure of the layer and the deformation of the roller. The necessary
pressure for dispensing the toner is preferably supplied by a small pump
(not shown) which pumps the toner from a toner container (not shown) at a
preset pressure. Any suitable pump and any suitable container known in the
art may be used for this purpose, as well as any other suitable means for
providing the desired toner pressure (such as a pressured tank containing
the desired liquid toner). Preferably, surplus toner unabsorbed by layer
68 returns to the container for reuse.
In a preferred embodiment of the present invention, developer roller 22 and
applicator roller 65 rotate in the same rotational direction (indicated by
arrows 13 and 70 in FIG. 2, respectively), such that their surfaces move
in opposite directions. In this embodiment, applicator roller 65 is
operative to scrub surface 21 and to remove the residual toner (which
normally remains after the selective transfer of toner to surface 16) on
surface 21 while applying a new, homogeneous, toner layer to the surface
electrophoretically. More specifically, in a preferred embodiment, the
absolute velocity of surface 64 is preferably approximately 2-3 times
greater than that of surface 21. The relative motion of the applicator and
the developer surface is between three to four times the absolute velocity
of the developer surface.
In the absence of this scrubbing action and if toner remaining on the
developer roller after development of the image is not removed, the DMA on
the development roller will be uneven, with up to 30% variations.
Furthermore, the toner which remains on the developer roller after
development of the image is highly concentrated, and not easily removed,
especially if a sponge roller is used for cleaning, so as not to damage
surface 21.
For the preferred scrubbing type application, as surfaces 21 and 64 merge
into contact at surface merge line 74, the residual toner is scraped off
surface 21 by the open cells of surface 64, due to the substantial
relative motion between the surfaces and the pressure applied by spring
72. During the relatively long period of surface engagement, the scraped
toner is indistinguishably mixed with the new toner carried by surface 64,
and a homogeneous layer of toner remains on surface 21 as the surfaces
disengage at divergence line 76.
Roller 65 is preferably electrified by a D.C. source, to a different
voltage than that of surface 21, in order to induce electrophoretic
transfer of toner particles from roller 65 to surface 21. The physical
contact between the surfaces is operative more to squeegee and homogenize
the applied layer of toner, rather than to apply the layer of toner, which
is applied, as aforesaid, by electrophoresis. In general, the solids
concentration of the toner rises considerably during the electrostatic
application of the toner to surface 21.
In a preferred embodiment of the invention, a liquid toner of 5%-10% solids
concentration is supplied by dispenser 62 to roller 65. After application
onto the developer roller, the layer has a solids concentration of between
15 and 20 percent.
When a more concentrated toner is supplied by dispenser 62, applicator 65
may further or alternatively be connected to an a.c voltage source, which
is operative to somewhat reduce the viscosity of the toner and generally
to cause the deposition of a smoother layer on surface 21 of developer
roller 22.
In general it is desirable that the liquid toner layer which develops the
latent image have as high a solids concentration as possible, preferably
30-50%.
In a preferred embodiment of the invention, developer assembly 23 further
includes a squeegee roller 78 in operative contact with roller 22
downstream of dispenser roller 65 and before roller 22 contacts drum 10.
Preferably squeegee roller 78 is electrified with a voltage comparable
with that of applicator 65, such that the outer surface of the squeegee
repels the charged particles of the toner layer on surface 21. Squeegee
roller 78 is also preferably resiliently urged against roller 22 such that
liquid carrier is removed from the layer as it passes the squeegee roller.
The mechanical pressure and the electric repulsion of roller 78 are
operative to squeegee the layer of toner, so that the layer is more
condensed and uniform as the layer reaches image carrying surface 16. By
adjusting the mechanical pressure and by biasing the roller to an
appropriate voltage, the concentration of the toner layer can be adjusted
to a desirable level.
Thus, in a preferred embodiment, the liquid toner is supplied to roller 78
at a concentration less than that required for optimal development of the
latent image. When roller 78 is urged against surface 21 of developer
roller 22, it mechanically removes excess toner fluid from the layer
impressed on surface 21, and when charged with a suitable electric
potential, it repels the charged toner particles and causes them to more
closely adhere to surface 21. The excess fluid which has been removed is
recovered for reuse. The solids content of the layer is mainly a function
of the mechanical properties of the rollers and of the applied voltages
and pressures and is only slightly influenced by the initial concentration
for a considerable range of initial toner concentrations.
In a preferred embodiment of the invention, squeegee roller 78 comprises an
aluminum core which is anodized and coated with a thin layer
(approximately 50 micrometers) of polyurethane.
In principle, the system described above does not require that the portions
of the toner layer that have not been transferred to drum 10 in the
development of the latent image be removed from developer roller 22
between cycles. However, the inventors have found, that the toner
uniformity can be further improved if residual toner on the developer
roller is removed between coating cycles. For this purpose a cleaning
station 82 may be provided, which may comprise a sponge or a brush or
similar apparatus, to remove the excess toner concentrate from surface 21
of developer roller 22. The toner so removed may then be pumped back for
reuse, after mixture with fresh toner, through dispenser 62 into the
sponge of applicator 65.
Cleaning station 82 preferably comprises a sponge roller 84, which is
preferably formed of a resilient open cell material similar to that of
layer 68 of roller 65. Roller 84 is situated such that it resiliently
engages a portion of surface 21 between the transfer area (i.e. the area
of surface 21 engaged by surface 16 ) and the application area (i.e. the
area of surface 21 engaged by surface 64), thereby removing residual toner
from surface 21 before the application of new toner. In a preferred
embodiment of the invention, sponge 84 may be supplied with toner carrier
liquid which may assist in cleaning surface 21 by loosening and carrying
away the residual toner particles scraped off the surface.
An alternative preferred embodiment of a cleaning system 110, especially
suitable for removing residual toner from the developer roller, is shown
in FIG. 3. Cleaning 110 comprises a tandem roller arrangement in which a
first, biased squeegee, roller 112 removes residual toner particles from
developer roller 22 by electrostatic transfer. To this end roller 112 is
biased to a voltage that, in conjunction with the developer roller
voltage, causes the charged toner particles to be attracted to roller 112.
The general construction of roller 112 is preferably, similar to that
described above for roller 78.
A second, sponge cleaning, roller 114, removes the toner particles from the
biased squeegee roller 112 by abrasion, preferably aided by an
electrostatic field. The toner is removed from the sponge roller by a
blade 116 which indents the surface of the sponge and scrapes the material
which is extruded away. In certain circumstances, depending on the toner
material used, one or both of the squeegee-developer or squeegee-sponge
surfaces is wetted by a cleaning liquid or dilute toner to aid in transfer
and removal of the toner particles.
The toner removed by any of the methods is preferably recycled and used for
recoating the developer roller.
It has been found that such cleaning, even if it is not perfect, tends to
reduce or eliminate any "memory" effects on the development roller.
Cleaning station 82 may be especially useful in the event the toner is of
a type which becomes discharged by the electric fields in the interface
between the surfaces of developer roller 22 and drum 10.
The latent image developed by means of the process described above may be
directly transferred to a desired substrate from the image forming surface
in a manner well known in the art. Alternatively, as shown in FIG. 1,
there may be provided an intermediate transfer member 40, which may be a
drum or belt and which is in operative engagement with photoreceptor
surface 16 of drum 10 bearing the developed image. Intermediate transfer
member 40 rotates in a direction opposite to that of photoreceptor surface
16, as shown by arrow 43, providing substantially zero relative motion
between their respective surfaces at the point of image transfer.
Intermediate transfer member 40 is operative for receiving the toner image
from photoreceptor surface 16 and for transferring the toner image to a
final substrate 42, such as paper. Disposed internally of intermediate
transfer member 40 there may be provided a heater 45, to heat intermediate
transfer member 40 as is known in the art. Transfer of the image to
intermediate transfer member 40 is preferably aided by providing
electrification of intermediate transfer member 40 to provide an electric
field between intermediate transfer member 40 and the image areas of
photoreceptor surface 16. Intermediate transfer member 40 preferably has a
conducting layer 44 underlying an elastomer layer 46, which is preferably
a slightly conductive resilient polymeric layer.
Various types of intermediate transfer members are known and are described,
for example in U.S. Pat. No. 4,684,238, PCT Publication WO 90/04216 and
U.S. Pat. No. 4,974,027, the disclosures of all of which are incorporated
herein by reference.
Following the transfer of the toner image to substrate 42 or to
intermediate transfer member 40, photoreceptor surface 16 engages a
cleaning station 49, which may be any conventional cleaning station. A
scraper 56 completes the removal of any residual toner which may not have
been removed by cleaning station 49. A lamp 58 then completes the cycle by
removing any residual charge, characteristic of the previous image, from
photoreceptor surface 16.
The cleaning system shown in FIG. 3 is also used, in a preferred embodiment
of the invention, in place of cleaning station 49. In such case, a
somewhat softer and more conformal roller is used so as to insure good
contact between the squeegee roller 112 and the image forming surface.
In an alternative embodiment of the invention, reversal transfer is used.
In this embodiment, the desired image is formed by the areas of toner
concentrate which remain on surface 21 of developer roller 22 after the
development of photoreceptor surface 16, and developer roller 22 and not
drum 10 which is then brought into operative association with an
intermediate transfer member or a final substrate so as to obtain a print
of the desired image. Any preferred embodiment of the developer assembly
described above may also be used in the context of this embodiment.
In a further preferred embodiment of the invention, applicator assembly 65
may be replaced by a curved, electrified plate developer situated parallel
to the surface of developer roller at about 6 o'clock on the developer
roller. Liquid toner at about 1-10% solids concentration preferably,
between 5-10% concentration is fed into the region between the plate and
the developer roller and plates the developer roller with a layer having
approximately 5-20% toner solids, preferably, 15-20% toner solids. The
squeegee roller is then operative to further concentrate the layer to the
desired concentration for developing the latent image. In this embodiment,
if cleaning station 49 is provided, a toner layer having a .+-.5%
uniformity is presented to the image forming surface.
A preferred, but by no means limiting, liquid toner material for use in the
present invention is prepared in the following manner:
Compounding
865.4 g of Surlyn 1605 Ionomer (DuPont), 288.5 g Mogul-L carbon black
(Cabot), 28.8 g Endurophtal blue BT583D (Cookson) and 17.3 g aluminum
stearate (Merck) are blended for 20 minutes on a two roll mill heated to
150.degree. C. until the blend is homogeneous. The blended material is
removed from the mill and shredded in preparation for the next step.
Plasticizing
2 Kg of the blended material and 2 Kg of Marcol 82 (EXXON) are heated for
one hour in a double planetary mixer, without mixing. The material is then
mixed for 45 minutes at low speed and for 30 minutes at high speed. The
material is discharged, while still warm, from the mixer, shredded and
ground in a cooled meat grinder in preparation for grinding.
Grinding-Size Reduction
690 g of plasticized material is ground together with 1610 g of Marcol 82
in an attritor charged with 3/16" carbon steel balls. The material is
ground at 250 RPM for 30 hours at 55.degree. C..+-.3.degree. C. The
resulting material is diluted to 10% non-volatile solids (NVS) content and
screened through a 300 micrometer screen to remove unground particles. Any
metalic contaminating material in the toner is removed by magnetic
treatment and the resulting material is charged with charge director to
form a 10% NVS liquid developer, suitable for the present invention.
It will be appreciated by persons skilled in the art that the present
invention is not limited to what has been particularly shown and described
hereinabove. Rather, the scope of the present invention is defined only by
the claims that follow:
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